Multiple focus acoustic lens transducer combinations for high intensity focused ultrasound treatment of solid tumours

Clinical trials of high intensity focused ultrasound (HIFU) treatments of solid tumours have produced promising results. The major problem currently faced by the use of HIFU is that individual thermal lesions generated by conventional spherical transducers are much smaller than the target. Therefore, a large number of lesions must be formed to treat the target, leading to excessively long treatment times. To address this problem, I designed and constructed a 9-focus Lens Transducer Combination (LTC) prototype, and used it to create large thermal lesions in porcine kidney tissues ex vivo and in vivo. The results demonstrated that the lens can withstand the power sufficient for creating lesions in highly perfused tissues and the lesions were much larger than those created by the spherical transducer alone. I also designed and constructed a 9-focus beam-steering LTC prototype that produced the foci off the central axis of the transducer, and used this prototype to heat transparent tissue-mimicking phantom in water. During heating, the lens was continuously rotated and larger lesions were created compared to those created using the former, non-beam-steering lens prototype. I developed a 3D numerical model to calculate the size and shape of thermal lesions created by LTCs. Discrepancies existed between the predicted and those observed experimentally, likely due to unrealistic assumptions adopted in the numerical model. I also developed a model, employing the back-projection method, to measure the quality of the transducers used for multi-focus LTCs. The ability of the model to identify transducer defects was demonstrated using three transducers of various geometries and defects. The defects detected by this model were taken into account in the calculation of the intensity distribution of the LTCs. The resulting distributions agreed better with the measurements than those calculated assuming ideal transducers. I developed a 2D numerical model to study the effects of intervening tissues on ultrasound beam propagation. The results demonstrated that the distortions to the 3-focus field of a LTC were modest, including small spatial shifts and tilts of the focal zone, when the intervening tissues were of uniform thickness and at small oblique angles to the transducer beam axis.